19,584 research outputs found
Scalar glueball and meson spectroscopy in unquenched lattice QCD with improved staggered quarks
We present results of an exploratory study of singlet scalar states in
unquenched QCD using both glueball and meson operators. Results for non-singlet
non-strange scalar mesons are also presented. We use Asqtad improved staggered
fermions and gauge configurations generated by the MILC collaboration at
lattice spacings of .12 and .09 fm. In this formulation, the glueball mass is
not significantly different from the quenched value at finite lattice spacing.
Significant taste violations are present in the scalar sector. At light quark
masses, decay channels complicate the mass determinations. There is some
evidence that the non-strange singlet meson lies below the non-singlet meson.Comment: Lattice 2005 (hadron spectrum and quark masses), 6 pages, 4 figure
Pseudoscalar singlet physics with staggered fermions
We report on progress in measuring disconnected correlators associated with
pseudoscalar flavor-singlet mesons. This will eventually allow us to compute
the masses of the eta and eta' mesons. Flavor-singlet physics also presents an
interesting test of the staggered fermion formulation, as disconnected
correlators are sensitive to whether the same action governs both sea quarks
and valence quarks. It can also help test the validity of the ``fourth-root
trick'' used in unquenched lattice calculations where the number of flavors
.Comment: Talk presented at Lattice 2005 (Hadron spectrum and quark masses), 6
pages, 3 figure
Embryo impacts and gas giant mergers II: Diversity of Hot Jupiters' internal structure
We consider the origin of compact, short-period, Jupiter-mass planets. We
propose that their diverse structure is caused by giant impacts of embryos and
super-Earths or mergers with other gas giants during the formation and
evolution of these hot Jupiters. Through a series of numerical simulations, we
show that typical head-on collisions generally lead to total coalescence of
impinging gas giants. Although extremely energetic collisions can disintegrate
the envelope of gas giants, these events seldom occur. During oblique and
moderately energetic collisions, the merger products retain higher fraction of
the colliders' cores than their envelopes. They can also deposit considerable
amount of spin angular momentum to the gas giants and desynchronize their spins
from their orbital mean motion. We find that the oblateness of gas giants can
be used to infer the impact history. Subsequent dissipation of stellar tide
inside the planets' envelope can lead to runaway inflation and potentially a
substantial loss of gas through Roche-lobe overflow. The impact of super-Earths
on parabolic orbits can also enlarge gas giant planets' envelope and elevates
their tidal dissipation rate over 100 Myr time scale. Since giant
impacts occur stochastically with a range of impactor sizes and energies, their
diverse outcomes may account for the dispersion in the mass-radius relationship
of hot Jupiters.Comment: 19 pages, 7 figures, 7 tables. Accepted for publication in MNRA
Glueball mass measurements from improved staggered fermion simulations
We present the first 2+1 flavour spectrum measurements of glueball states
using high statistics simulations with improved staggered fermions. We find a
spectrum consistent with quenched measurements of scalar, pseudoscalar
andtensor glueball states. The measurements were made using 5000 configurations
at a lattice spacing of 0.123 fm and pion mass of 280 MeV and 3000
configurations at 0.092 fm with a pion mass of 360 MeV. We see some evidence of
coupling to 2 pion states. We compare our results with the experimental
glueball candidate spectrum as well as quenched glueball estimates.Comment: 22 pages, 19 figures and 8 tables, minor additions on mixing
post-refere
X-ray Bursts from the Accreting Millisecond Pulsar XTE J1814-338
Since the discovery of the accreting millisecond pulsar XTE J1814-338 a total
of 27 thermonuclear bursts have been observed from the source with the
Proportional Counter Array (PCA) onboard the Rossi X-ray Timing Explorer
(RXTE). Spectroscopy of the bursts, as well as the presence of continuous burst
oscillations, suggests that all but one of the bursts are sub-Eddington. The
remaining burst has the largest peak bolometric flux of 2.64 x E^-8
erg/sec/cm^2, as well as a gap in the burst oscillations, similar to that seen
in Eddington limited bursts from other sources. Assuming this burst was
Eddington limited we obtain a source distance of about 8 kpc. All the bursts
show coherent oscillations at the 314.4 Hz spin frequency. The burst
oscillations are strongly frequency and phase locked to the persistent
pulsations. Only two bursts show evidence for frequency drift in the first few
seconds following burst onset. In both cases the initial drift corresponds to a
spin down of a few tenths of a Hz. The large oscillation amplitude during the
bursts confirms that the burst flux is modulated at the spin frequency. We
detect, for the first time, a significant first harmonic component in burst
oscillations. The ratio of countrate in the first harmonic to that in the
fundamental can be > 0.25 and is, on average, less than that of the persistent
pulsations. If the pulsations result from a single bright region on the
surface, the harmonic strength suggests the burst emission is beamed, perhaps
due to a stronger magnetic field than in non-pulsing LMXBs. Alternatively, the
harmonic content could result from a geometry with two bright regions.Comment: AASTeX, 15 pages, 4 figures. Accepted for publication in the
Astrophysical Journal Letter
CO Line Emission and Absorption from the HL Tau Disk: Where is all the dust?
We present high-resolution infrared spectra of HL Tau, a heavily embedded
young star. The spectra exhibit broad emission lines of hot CO gas as well as
narrow absorption lines of cold CO gas. The column density for this cooler
material (7.5+/-0.2 x 10^18 cm-2) indicates a large column of absorbing gas
along the line of sight. In dense interstellar clouds, this column density of
CO gas is associated with Av~52 magnitudes. However, the extinction toward this
source (Av~23) suggests that there is less dust along the line of sight than
inferred from the CO absorption data. We discuss three possibilities for the
apparent paucity of dust along the line of sight through the flared disk: 1)
the dust extinction has been underestimated due to differences in circumstellar
grain properties, such as grain agglomeration; 2) the effect of scattering has
been underestimated and the actual extinction is much higher; or (3) the line
of sight through the disk is probing a gas-rich, dust-depleted region, possibly
due to the stratification of gas and dust in a pre-planetary disk.Comment: To be published in The Astrophysical Journa
Airborne bacterial populations above desert soils of the McMurdo Dry Valleys, Antarctica
Bacteria are assumed to disperse widely via aerosolized transport due to their small size and resilience. The question of microbial endemicity in isolated populations is directly related to the level of airborne exogenous inputs, yet this has proven hard to identify. The ice-free terrestrial ecosystem of Antarctica, a geographically and climatically isolated continent, was used to interrogate microbial bio-aerosols in relation to the surrounding ecology and climate. High-throughput sequencing of bacterial ribosomal RNA (rRNA) genes was combined with analyses of climate patterns during an austral summer. In general terms, the aerosols were dominated by Firmicutes, whereas surrounding soils supported Actinobacteria-dominated communities. The most abundant taxa were also common to aerosols from other continents, suggesting that a distinct bio-aerosol community is widely dispersed. No evidence for significant marine input to bio-aerosols was found at this maritime valley site, instead local influence was largely from nearby volcanic sources. Back trajectory analysis revealed transport of incoming regional air masses across the Antarctic Plateau, and this is envisaged as a strong selective force. It is postulated that local soil microbial dispersal occurs largely via stochastic mobilization of mineral soil particulates
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